This invention relates to a four-wheel drive transmission device which is applied particularly to passenger motor vehicles.
Numerous four-wheel drive devices are known which comprise a mechanism box at the outlet of the transmission to transmit the movement to the front axle and to the rear axle. Each axle has a differential placed between the wheels, and which receives the movement from the mechanism box. Certain devices offer, in addition, the presence of a differential between the front axle and the rear axle, this differential being placed directly at the outlet of the mechanism box. So as not to increase considerably the consumption of fuel, these mechanisms make it possible to function either in two-wheel drive or in four-wheel drive. For this reason, a control available to the driver makes it possible to go into four-wheel drive. These devices exhibit the drawback, when they are in two-wheel drive, of presenting a risk of loss of road adhesion by a wheel. In this case, the driver of the vehicle must very quickly lock the transmission of the vehicle into four-wheel drive. This clutching maneuver must be made when stopped or at very low speed. Consequently, the manipulation of these four-wheel drive devices is very difficult, and in addition it is very difficult to perform it at the best moment. Finally, this type of transmission requires the constant attention of the driver of the vehicle, which is achieved at the expense of driving safety.